Operational amplifiers are important building blocks and, as such, they are widely used in many analog signal processing systems. In many applications it is convenient to use a fully differential operational amplifier, that is to say an amplifier having a differential output. As is known to those skilled in the art, a fully differential operational amplifier requires a common mode feedback circuit to operate properly. The article entitled "A family of Differential NMOS Analog Circuit for PCM Code Filter Chip," IEEE JSSC, December 1982, for example, is a review of this type of circuit.
In substance, as it is shown in FIG. 1, a fully differential amplifier amplifies a differential input signal (Vid) and provides a differential output signal (Vod). The fully differential amplifier is also provided with an auxiliary input (INCM) through which control of the common mode at the outputs nodes (Vcm) is applied to set it exactly half-way between the voltages of the supply rails (V.sub.DD and V.sub.SS). In single-stage fully differential amplifiers, the common mode feedback circuit may be considered extremely efficient as it does not require any significant current consumption. As indicated in a simplified manner in FIG. 2, V.sub.R and V.sub.B are two reference voltages, which can respectively be equal to about (V.sub.DD +V.sub.SS)/2 and V.sub.GSm3.
On the other hand, in those applications where it is essential to ensure the maximum output voltage swing (ideally equal to the supply voltage), like for instance in systems which function at low and extremely low supply voltages, it becomes necessary to employ at least a two-stage, fully differential amplifier. In these cases, the common mode feedback circuit cannot be realized the same as for a single-stage amplifier according to the scheme shown in FIG. 2, because two inversions would occur between the feedback node INCM and the output nodes. This would result in a positive feedback that would make the system unstable.
Up until now, the conventional approach for realizing a common mode control loop in a two-stage amplifier has been that of adding an inverting stage to the feedback circuit of FIG. 2, to have three inversions in the common mode feedback circuit, according to a scheme as shown in FIG. 3. However, this approach is inefficient because of the additional current consumption that is not even minimally exploited by the differential signal, since the differential signal practically does not "see" this additional inverting stage. Especially in low voltage, battery-operated systems it is important to minimize the current consumption.